1. Field of the Invention
The present invention relates to a wholly aromatic polyamide fiber non-woven sheet and processes for producing the same. More particularly, the present invention relates to a wholly aromatic polyamide fiber non-woven sheet having a high density, an enhanced impregnating property, and a satisfactory surface smoothness and processes for producing the same.
2. Description of the Prior Art
Polyester, nylon, and other thermoplastic synthetic fibers bonded or entangled with each other are used for various types of non-woven sheets on the market. These thermoplastic synthetic fibers are advantageous in that they are industrially produced and, thus, are readily available and in that their thermoplasticity allows the use of conventional bonding methods, for example, thermocompression bonding, in the non-sheet production process. The same thermoplasticity, however, has a great adverse effect on the thermal properties of the non-woven sheets. That is, the resultant non-woven sheet exhibits poor heat resistance and flame retardancy and, thus, is not suitable for use as a lightweight composite material, such as building material, interior material, electrical insulating material, on honeycomb cone, which require high heat resistance and flame retardancy.
Aromatic polyamides are known materials with high heat resistance and flame retardancy. However, aromatic polyamides are generally non-thermoplastic and, thus, cannot be readily shaped into a paper-like sheet. Several techniques have been heretofore developed to utilize aromatic polyamides as a paper-like sheet, however, the products resulting from these techniques still leave much to be desired with regard to their properties.
Aromatic polyamide paper-like sheets known hitherto may be roughly classified into the following three groups:
(1) Paper-like sheets in which a portion of the aromatic polyamide fibers is in the special form of fibrids having a specific entangling property. This type of sheet is prepared by a process as typically disclosed in Japanese Examined Patent Publication (Kokoku) No. 35-11851 or U.S. Pat. No. 2,999,788 or 3,123,518.
(2) Non-woven sheets in which a thermoplastic material, for example, a polyester, is used as a binder;
(3) Non-woven sheets in which at least a part of the aromatic polyamide fibers used is not substantially crystallized and oriented and the polyamide fibers are heat-bonded under pressure at a temperature above the glass transition point of the noncrystallized and oriented polyamide fibers but below the glass transition point of the crystallized and oriented polyamide fibers, which sheet is prepared by a process as typically disclosed in Japanese Unexamined Patent Publication (Kokai) No. 51-75179.
Conventional sheets of these three groups, however, all have serious problems with regard to their properties in practical use, i.e., structural density, impregnating property, and heat resistance, and, thus, are still unsatisfactory.
Products of group (1) have a sufficiently dense structure and an excellent surface smoothness because of the use of a material having the special form of fibrids, but have a poor impregnating property. The poor impregnating property reduces the useful life of the sheet and results in unsatisfactory dielectric strength and mechanical strength when used for an insulating material essentially requiring the use of an insulating oil, an insulating varnish, and the like and a lightweight composite material and an electrical material, both of which require essentially a resin impregnation treatment. The characteristics of dense structure, smooth surface, but poor impregnating property are inherent in products in which fibrids are used. Therefore, it is considered to be very difficult to improve only the poor impregnating property of the product, while keeping its excellent denseness and surface smoothness. That is, the product is in the form of highly developed fibrids on thin film and it is considered, thus, that the fibrids have a high entangling ability to unite aromatic polyamide fibers into a sheet. Therefore, if the content is increased, the structural density and the surface smoothness of the resultant sheet are enhanced, while air bubbles are formed by the fibrids and a cover is formed over the pores penetrating through the thickness of the sheet at both surfaces thereof, resulting in voids isolated from each other in the sheet. The presence of the voids is a major cause for the poor impregnating property and unsatisfactory dielectric strength of the sheet impregnated with a resin. Decreasing the pulp content will improve the impregnating property of the resultant sheet, but, at the same time, will reduce the density and surface smoothness. As a products of group (1) on the market, there may be mentioned Nomex Type 410, intended for electrical insulating material, and Nomex Type 424, intended for an impregnating matrix, both products being manufactured by E. I. du Pont de Nemours & Co., Inc. If the porosity described hereinafter is used as a measure of the denseness and an air permeability rate (the time, in seconds, required for 100 cc of air to pass through a sheet) is used as a measure of the impregnating property, the product of Nomex Type 410 exhibits a porosity of from 20% to 42% and, thus, has a dense structure, while the air permeability rate thereof is a very high value of about 10.sup.4 sec/100 ml, indicating the poor impregnating property of the product.
The cross-sectional profile of this type of sheet, observed under a scanning electron microscope at a magnification of 1000 is shown in FIG. 1. It is clearly confirmed from FIG. 1 that isolated voids are present in the sheet. Therefore, this sheet is estimated to have a high pulp content. On the other hand, the Nomex Type 424 sheet is estimated to have a decreased fibrid content and to exhibit an improved impregnating property because it exhibits an air permeability rate as low as 1 to several seconds/100 ml, while the porosity thereof is as high as 65%, indicating the highly porous structure of the sheet. That is, the products of group (1) cannot essentially exhibit an adequate impregnating property while retaining a dense structure. This feature is considered to be a major cause for the fact that the product can only exhibit unsatisfactory functions when it is used for producing a lightweight composite material such as honeycomb core and an impregnation type electrical insulating material requiring resin impregnation.
Products of group (2) have the essential disadvantage that the excellent heat resistant characteristic of the aromatic polyamide is damaged because a thermoplastic material having a low heat resistance is used as the binder. As products of group (2) on the market, there may be mentioned actually manufactured heat-resistant non-woven sheets. These non-woven sheets are all considered to be aromatic polyamide non-woven sheets containing polyethylene terephthalate fibers as the binder. For the above-mentioned reason, the content of the thermoplastic material in the sheet should be controlled to the minimum level required to form the sheet. Therefore, the sheet inevitably tends to exhibit a reduced denseness. As a result of measurements on heat-resistant non-woven sheets collected from the market, the present inventors found that the porosity is in the range of from 40% to 70% and the air permeability rate is in the range of from 0.1 to several seconds/100 ml. Therefore, these non-woven sheets exhibit an excessively large air permeability. Of course, the heat resistance of these non-woven sheets is significantly lower than that of a sheet consisting of an aromatic polyamide alone. Even if a little reduction in heat resistance is tolerated, the non-woven sheets can still exhibit only unsatisfactory functions due to their highly porous structure when they are used for the production of a lightweight composite material such as a honeycomb core and an impregnation type electrical insulating material requiring resin impregnation or the like.
Products of group (3) have not generally come out on the market yet. Since the raw material consists of only fibers having substantially no plasticity, the resultant sheet usually does not have a dense structure. This is presumed from the porosity thereof of 30% to 70% described in Japanese Unexamined Patent Publication (Kokai) No. 51-75179. Measurements by the present inventors invention, showed that the porosity is in the range of from 40% to 70% and the air permeability rate is in the range of from 0.1 to several seconds/100 ml. For this reason, products of group (3) can only exhibit unsatisfactory functions due to their highly porous structure when used for the production of a lightweight composite material such as a honeycomb core and an impregnation type electrical insulating material requiring resin impregnation or the like. The present inventors made extensive studies in order to develop a quite novel sheet having satisfactory structural denseness, adequate impregnating property and high heat resistance.